Underwater forest (4)
I believe the answer is:
kelp
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater forest</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for kelp that I've seen before include "Tough brown seaweed" , "Large seaweeds with leathery fronds" , "Type of finch" , "Large brown leathery seaweed" , "Brown seaweed with fluted leathery fronds" .)